Semiconductor memory devices are widely used in electronic circuits. Some examples of semiconductor memory devices are dynamic random access memory (DRAM) devices, static random access memory (SRAM) devices, and flash memory devices.
A semiconductor memory device may be classified as either a volatile memory device or a non-volatile memory device. A volatile memory device may lose data stored in the device when power is removed from the device. On the other hand, a nonvolatile memory device may retain its data even without power. Flash memory devices, which are a type of nonvolatile memory device, are frequently used for storing data. However, flash memory devices are not generally configured as random access memory devices. In addition, flash memory devices are disadvantageous in that the time required for reading or writing data from or to such devices may be relatively long.
In order to overcome some or all of these disadvantages, ferroelectric random access memory (FRAM) devices, magnetic random access memory (MRAM) devices, and phase-changeable random access memory (PRAM) devices have been developed.
A PRAM device may read or erase its data by using a phase-changeable material. When heat is applied to the phase-changeable material, the state of the phase-changeable material may be changed from a crystalline state to an amorphous state. When heat is removed from the phase-changeable material, the state of the phase-changeable material may be changed from the amorphous state back to a crystalline state. The phase-changeable material may generally be a chalcogenide.
PRAM devices having a highly integrated structure and/or capable of operating with a relatively low voltage have been developed. In particular, structural features of a PRAM device unit cell, or of an electrical circuit that is included in the PRAM device, may be changed to enable the PRAM device to be highly integrated. For example, a PRAM device may have a T-shape structure, a confined structure and/or an edge contact structure.
In addition, providing thermal insulation of the phase-changeable material and/or decreasing the size of the programming region of a PRAM device may enable the PRAM to operate with a relatively low power.
A PRAM device may operate as follows. Electrical current applied to a lower electrode may generate Joule's heat, which may change the phase of a layer of phase-changeable material so that data may be stored in the PRAM device. The PRAM device may read the data using a variation of the resistance of the layer of phase-changeable material due to a phase change of the layer of phase-changeable material.
Generally, the portion of the layer of the phase-changeable material where the phase change occurs may be referred to as the “programming region.” When the programming region has an amorphous state, the state of the PRAM device is referred to as the “reset state.” On the other hand, when the programming region has a crystalline state, the state of the PRAM device is referred to as the “set state.” The resistance of the portion of the programming region making contact with the lower electrode may be relatively high in the reset state. On the other hand, the resistance of the portion of the programming region making contact with the lower electrode is relatively low in the set state.
In order to allow a PRAM device to operate efficiently, the state of the portion of the programming region making contact with the lower electrode should switch rapidly between the amorphous and crystalline states with the aid of the electrical current applied to the lower electrode.
When the area of the programming region is relatively large, the current required to rapidly change the state of the programming region may also be large. Thus, in order to enable the PRAM device to operate with relatively low power, the area of the programming region may be reduced.
If the phase-changeable material layer is not thermally insulated, heat applied to the phase-changeable material layer may be easily dissipated. Thus, it may become necessary to apply a relatively large current to the lower electrode to change the phase of the phase-changeable material layer. As a result, it may be preferable to thermally insulate the phase-changeable material layer.
In addition, in order to enable the phase-changeable memory device to operate with the relatively low current, the reset current required to reset the phase-changeable memory device should be relatively small. Furthermore, the initial current required to initially activate the phase-changeable memory device should be relatively small. The initial activation of the phase-changeable memory device is referred to herein as a “first firing.”
In particular, it may be desirable for the initial current to be similar in magnitude to the normal current required for driving the PRAM device after the first firing. An example of such normal current is the reset current. However, in conventional devices, if the initial current is similar in magnitude to the normal current, the first firing may not occur.
The first firing will now be briefly described. The heat required for initial activation of a PRAM device may be substantially larger than that required for driving the PRAM device after the initial activation. Thus, when a PRAM device is initially activated, it may be necessary to apply a current to the lower electrode substantially larger than that required for driving the PRAM device after the initial activation in order to generate the heat required for the initial activation.
After the first firing, relatively low heat may be required to change the phase of the phase-changeable material layer in a PRAM device. Thus, a relatively low current may be applied to the lower electrode after the first firing.
Since a relatively large initial current may be applied to the lower electrode in the first firing, it may be necessary to apply a relatively high voltage to a source region of a transistor that supplies the initial current. Thus, the structure of the PRAM device should be capable of minimizing damage that may occur due to the relatively large current and high voltage required for the first firing. It may be difficult to manufacture a highly integrated PRAM device due to these constraints.
If structural changes of the unit cell or the electrical circuit are implemented to overcome the above-described problems, the time and cost required for changing existing facilities may be excessive. In addition, phase-changeable material in general has unique characteristics. Thus, development of phase-changeable materials is also difficult.